Efforts to improve the efficiency of photovoltaic devices, which are used to convert solar energy to electrical energy, are widely supported. The cost of producing electrical power from solar energy has continued to decline in recent years as a result of these efforts and the market for solar cells has increased as the cost of producing electrical power has declined. Most of the solar cells now on the market are made of silicon, but higher efficiency cells from other materials have been investigated in recent years. Particular interest has been focused on gallium arsenide and related alloys and, more recently, on indium phosphide and related alloys. One of the important properties of indium phosphide is its resistance to radiation damage. This property is particularly advantageous in solar cells for space application, but it leads to long-lived and efficient cells for terrestrial applications as well.
Means of concentrating solar energy onto solar cells for terrestrial applications are also being developed. Intensities of hundreds or even thousands of times that of the sun are considered feasible to apply to solar cells if heat dissipation is adequate and cells can be developed which are long-lived at such conditions.
Significant increases in solar cell efficiency are possible from the use of tandem subcells of different materials, the different materials having different energy bandgaps between their valence electron bands and their conduction bands. A tandem cell is described in U.S. Pat. No. 5,019,177, which is incorporated herein by reference. One of the subcells disclosed in this patent is comprised of indium phosphide. The growth of tandem cells by epitaxial methods in which the subcells are lattice-matched offers the possibility of monolithic structures having minimal defects or dislocations in the crystal lattice which lower the efficiency of the device. Tandem cells having indium phosphide as one of the subcells are particularly attractive, because there are a variety of III-V ternary and quaternary alloys available having differing energy bandgaps which are lattice-matched with indium phosphide. The integrated InP/InGaAs tandem solar cell has a theoretical efficiency greater than 30 per cent. Integrated cells having three or more subcells can also be considered, the top subcell having the highest energy bandgap, so as to absorb and convert the shortest wavelength light to electrical energy and allow the longer wavelength light to pass through, and the lowest subcell having the lowest energy bandgap to absorb the longer wavelength optical energy.
Lattice constants of compounds and alloys used to form photovoltaic cells are well-known. When such materials are combined in devices having subcells of the different materials, it is important that the lattice of the different materials have the same lattice constants to within a small difference. This avoids the formation of defects in the crystal structures which can drastically lower the efficiency of the devices. When the term lattice-matched is used herein, it denotes a difference in lattice constants of the materials of not more than about 0.3 per cent. Preferably, lattice constants are matched to within about 0.2 per cent.
In any tandem cell device having only two contacts, electrical connection must be made between the subcells. Preferably, these intercell ohmic contacts (IOCs) should cause very low loss of electrical power between cells. Therefore, they must have minimal electrical resistance. There are two methods known for making such IOCs--metal interconnects and tunnel junctions (or tunnel diodes). The metal interconnects can provide low electrical resistance, but they are difficult to fabricate, they result in complex processing, and can cause substantial loss in the device efficiency. Therefore, tunnel junctions are much preferred, because a monolithic integrated device can be produced having a plurality of subcells with tunnel junctions therebetween. But, the tunnel junctions must satisfy multiple requirements, such as low resistivity, low optical losses, and crystallographic compatibility through lattice-matching between top and bottom cell. Most importantly, they should exhibit a high peak current density. All these characteristics should be conserved upon completion of the entire monolithic device.
The use of In.sub.0.53 Ga.sub.0.47 As tunnel diodes for interconnecting tandem subcells of a solar cell has been proposed ("An In.sub.0.53 Ga.sub.0.47 As Tunnel Diode for Monolithic Multi-junction Solar Cell Applications," Proc. 20th IEEE Photovoltaic Specialist Conference, IEEE, New York, 1988, pp. 771-776). It was suggested that this material offered the possibility of low electrical resistance tunnel diodes, but optical loss could be substantial. Of course, tunnel diodes between subcells should absorb minimal amounts of the incoming optical energy, which allows the maximum amount of solar energy to be converted to electrical energy. It was further suggested that any tunnel junctions of this material be made very thin or be patterned, so as to minimize loss of solar energy in passing between subcells. The use of patterns, which decreases the area available for the tunnel junction, increases the need for a tunnel junction having low resistance and much higher peak current density.
Solar cells having indium phosphide as one of the subcells and indium gallium arsenide phosphide as a second subcell have been described ("Two-Terminal Monolithic InP/InGaAsP Tandem Solar Cells with Tunneling Intercell Ohmic Connections," Proc 22nd. IEEE Photovoltaic Specialists Conference, IEEE, New York, 1991, pp. 381-387). These cells were grown by the process of liquid phase epitaxy (LPE). It was found that, although In.sub.0.53 Ga.sub.0.47 As tunnel junctions could offer high peak current densities and low electrical resistance, such tunnel junctions could not be incorporated into tandem solar cells structures because melt-back problems during the LPE growth would not allow InP to be grown on the In.sub.0.53 Ga.sub.0.47 As tunnel junction. The tunnel junction to connect these subcells which could be grown by LPE, consisting of InGaAsP, had peak current and resistivity values far less favorable than attained with In.sub.0.53 Ga.sub.0.47 As tunnel junctions grown on an InP substrate.
There is great need for a tandem solar cell having indium phosphide or an indium phosphide alloy in the indium phosphide system as a subcell, another subcell which is lattice-matched to the indium phosphide and having an energy bandgap differing from that of indium phosphide, and a lattice-matched tunnel junction to interconnect the subcells. The tunnel junction should exhibit high peak current and low resistivity values so as to allow maximum efficiency of the tandem solar cell. There is also a need for a method to fabricate such a tandem solar cell as a monolithic device under conditions which will form and leave intact the tunnel junction when a subcell is grown on top of the tunnel junction.